The continuing search for methods to improve overall vehicle fuel economy has identified the torque converter or fluid coupling used between the engine and automatic transmission, as a relatively significant source of energy loss. Since the torque converter is a fluid coupling, it is not as efficient as a solid disk-type clutch. At any set of operating conditions (e.g., engine speed, throttle position, ground speed, transmission gear ratio), there is a relative speed difference between the driving and driven members of the torque converter. This relative speed differential represents lost energy which is dissipated from the torque converter as heat.
One method of improving overall vehicle fuel economy used by transmission builders is to build into the torque converter a clutch mechanism capable of "locking" the torque converter. "Locking" refers to eliminating relative motion between the driving and driven members of the torque converter so that little energy is lost in the fluid coupling. These "locking" or "lock-up" clutches are very effective at capturing lost energy at high road speeds. When they are used at low speeds, however, vehicle operation becomes rough and engine vibration is transmitted through the drive train. Rough operation and engine vibration are not acceptable to drivers.
The higher the percentage of time that the vehicle can be operated with the torque converter clutch engaged, the more fuel efficient the vehicle becomes. A second generation of torque converter clutches have been developed which operate in a "slipping" or "continuously sliding mode". These devices have a number of names, but are commonly referred to as continuously slipping torque converter clutches. The difference between these devices and lock-up clutches is that they allow some relative motion between the driving and driven members of the torque converter, normally a relative speed of 10 to 200 rpm. This slow rate of slipping allows for improved vehicle performance as the slipping clutch acts as a vibration damper. Whereas the "lock-up" type clutch could only be used at road speeds above approximately 50 mph, the "slipping" type clutches can be used at speeds as low as 25 mph, thereby capturing significantly more lost energy. It is this feature that makes these devices very attractive to vehicle manufacturers.
Continuously slipping torque converter clutches impose very exacting friction requirements on automatic transmission fluids (ATF's) used with them. The fluid must have a very good friction versus velocity relationship, that is, friction must always increase with increasing speed. If friction decreases with increasing speed, then a self-exciting vibrational state can be set up in the driveline. This phenomenon is commonly called "stick-slip" or "dynamic frictional vibration" and manifests itself as "shudder" or low speed vibration in the vehicle. Clutch shudder is very objectionable to the driver. A fluid which allows the vehicle to operate without vibration or shudder is said to have good "anti-shudder" characteristics. Not only must the fluid have an excellent friction versus velocity relationship when it is new, it must retain those frictional characteristics over the lifetime of the fluid, which can be the lifetime of the transmission. The longevity of the anti-shudder performance in the vehicle is commonly referred to as "anti-shudder durability". It is this aspect of performance that this invention addresses.
What we have now found is that fluids containing long chain alkyl phosphonates and metallic detergents provide significantly improved anti-shudder durability.